Only set VUE_VERSION if it is 3

What does this merge request do and why?

Both webpack and vite are not configured properly to handle an explicitly-set VUE_VERSION of 2. While we investigate and fix that, we can instead make sure that we only set VUE_VERSION when it is set to 3

How to set up and validate locally

1. Ensure Vite is enabled by adding the below configuration in gdk.yml, and set the vue version to 3:

---
vite:
  enabled: true
  vue_version: 3

1. Check out to this merge request's branch.
2. Run gdk reconfigure to check if regenerating all configuration is successful.
3. Do the same, but for a webpack block instead

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.